A cellular environment such as LTE (Long Term Evolution) standardized in 3GPP (3rd Generation Partnership Project) assumes to arrange a plurality of wireless base stations. Each wireless base station communicates with a wireless terminal in the local station communication area. This communication area is called a cell. The cell can be divided into a plurality of areas by imparting directivity to the antenna. Each divided area is called a sector cell. A cell will indicate a sector cell hereinafter.
In the LTE, a single wireless band (to be referred to as a band hereinafter) is generally used between the cells. Hence, each cell receives strong interference (to be referred to as a neighboring cell interference hereinafter) from a neighboring cell independently of whether the link is the up link or down link. In, for example, a down link, the communication path quality does not degrade for a wireless terminal near the wireless base station because the level difference between the desired signal received by the wireless base station and the interference signal from a neighboring cell is large. However, if signals are simultaneously transmitted to wireless terminals near a cell boundary using a single band in neighboring cells, the communication quality largely degrades because the level difference between the desired signal and the interference signal from a neighboring cell is small. This also applies to an up link.
In the LTE, ICIC (Inter-cell Interference Coordination) is expected to be applied as a related technique of solving the problem of neighboring cell interference (see, for example, non-patent literature 1). Non-patent literature 1 describes that the ICIC aims at controlling interference between neighboring cells, and information such as a resource use state or traffic load of another cell needs to be taken into consideration. One method of implementing the ICIC is FFR (Frequency Fractional Reuse).
The basic operation of FFR will be described. First, a priority band is set for each cell such that the band changes between neighboring cells. The wireless terminal reports communication path quality information to the wireless base station. The wireless base station determines using the communication path quality information whether the wireless terminal is a wireless terminal (to be referred to as a center wireless terminal hereinafter) little affected by the neighboring cell interference or a wireless terminal (to be referred to as an edge wireless terminal hereinafter) largely affected by the neighboring cell interference. Upon determining that the wireless terminal is an edge wireless terminal, the allocatable band is restricted to the priority band of the local cell. For a center wireless terminal, the allocatable band is not restricted. A scheduler allocates a wireless resource from allocatable bands to each wireless terminal in accordance with the communication path quality (see, for example, non-patent literature 2). Setting the priority bands such that they do not overlap between the neighboring cells allows to suppress the neighboring cell interference. For this reason, the throughput of the edge wireless terminal is expected to be higher as the communication path quality of the priority band is improved.
The priority band may dynamically be set. As a method of notifying the priority band between wireless base stations, load information is defined (see, for example, non-patent literature 3). A down link can be notified by RNTP (Relative Narrowband Tx Power), and an up link can be notified by HII (High Interference Indication). Notification information by RNTP or HII is created for each PRB (Physical Resource Block) number that is the minimum user channel band allocation unit. For example, for the PRBs of a priority band, RNTP is set to 1. Non-patent literature 3 describes that “1 indicates that the transmission power is not guaranteed”, and a PRB number that does not guarantee considering interference to the neighboring cell is notified.
FIG. 14 is an explanatory view showing a wireless resource setting operation in the related technique. FIG. 15 shows an example of priority band allocation. FIG. 16A shows an example of wireless resources allocatable to a wireless terminal UE1. FIG. 16B shows an example of wireless resources allocatable to a wireless terminal UE2.
Referring to FIG. 14, a wireless base station BS1 manages a cell C11, a wireless base station BS2 manages a cell C21, and a wireless base station BS3 manages a cell C31 as the local station communication area. Although each of the wireless base stations BS1, BS2, and BS3 can manage a plurality of cells, only one cell is illustrated for the sake of simplicity. The circles represent the effective ranges of radio waves based on the directivity of the antenna for the wireless base stations BS1, BS2, and BS3. Cell boundaries exist in the ranges where the circles overlap.
In the example shown in FIG. 14, two wireless terminals exist. Of these, the wireless terminal UE1 belongs to the cell C11. The wireless terminal UE1 is an edge wireless terminal that has the cell C31 as a neighboring cell. The wireless terminal UE2 is a center wireless terminal belonging to the cell C31. In LTE, the bands to be allocated to a single wireless terminal have an identical transmission power. As shown in FIG. 15, the allocatable bands are divided into three bands f1, f2, and f3, which are set as the priority bands of the cells C11, C21, and C31, respectively. Each priority band is formed from three PRBs.
In FIG. 14, since the edge wireless terminal UE1 exists in the cell C11, the wireless base station BS1 notifies the wireless base station BS3 of the neighboring cell C31 of RNTP=1 concerning the band f1. At this time, since no RNTP is received in the cell C11, neither the band nor the transmission power is restricted for the edge wireless terminal UE1, as shown in FIG. 16A. On the other hand, the wireless base station BS3 that has received the RNTP notification decreases the transmission power of the band f1 of the cell C11 to suppress interference of the cell C31 on the neighboring cell. For this reason, the wireless terminal UE2 decreases the transmission power by ΔE, and all bands are allocatable.